1. Field of the Invention
The present invention relates generally to a television receiver of projection type and is directed more particularly to a color television receiver of the projection type which can avoid without causing distortion in an image that a color phosphor pattern of a phosphor face of a color cathode ray tube is projected on a screen conspicuously.
2. Description of the Prior Art
In the art, a television receiver of a projection type using a color cathode ray tube of a single tube type is theoretically formed as depicted in FIG. 1.
In FIG. 1, reference numeral 1 generally indicates a color cathode ray tube whose phosphor face 1a is made as a color phosphor pattern of stripes. The image light on the phosphor face 1a of the color cathode ray tube 1 is projected through a projection lens 3 on a screen 4 to form a magnified image 5. In this case, at the same time when the magnified image 5 is formed on the screen 4, as shown in FIG. 2, the pattern of stripe-shaped color phosphors (color stripe pattern) forming the phosphor face 1a is formed on the screen 4 in a magnified scale, which is conspicuous, so that the magnified image 5 on the screen 4 is difficult to be seen.
In the prior art, in order to make the color stripe pattern inconspicuous on the screen 4, such a method has been proposed in which, as shown in FIG. 3, between the projection lens 3 and the color cathode ray tube 1 and near the projection lens 3 there is provided a light path splitting means, for example, a prism 6 having a given inclination angle .theta.. In this case, the image light emitted from a point P on the phosphor face 1a of the color cathode ray tube 1 is split in its light path into two paths by one and other prism faces 6a and 6b of the prism 6 and then projected on the screen 4 at two points Pa and Pb. As a result, as shown in FIG. 4, on the screen 4 there are formed two magnified images 5a and 5b which are parallely moved little in the horizontal direction within a region less than a alignment pitch L.sub.C of, for example, one set of color stripes and resultantly these magnified images 5a and 5b are synthesized as a magnified image 5 on the screen 4. In this case, the number of the color stripes of the magnified image 5 is twice that of the original color phosphor stripes so that the color stripe pattern becomes inconspicuous on the screen 4.
According to the prior art method shown in FIG. 3, however, the following defect is caused. That is, as shown in FIG. 5 by the broken lines, the incident angles of the image lights from a central point S and peripheral points L and R on the phosphor face 1a of the color cathode ray tube 1 to a predetermined point K on the prism face of the prism 1 are always different and hence the displacement of the image on the screen 4 (not shown in FIG. 5) is always different between the central portion and the peripheral portions of the screen 4 so that distortion is generated in the magnified images 5a and 5b on the screen 4.
In order to explain the above further in detail, with reference to FIG. 6 such a case will be now described in which the light from a predetermined point on the screen 4 (not shown in FIG. 6) is conducted through the prism 6 to the phosphor surface 1a.
In FIG. 6, it be assumed that the refractive index of the substance forming the prism 6 at the side of the screen 4 is taken as n1 and the refractive index of the same at the side of the phosphor face 1a as n2(&lt;n1).
As shown in FIG. 6A, if the light from the screen 4 is incident on the prism 6, where the inclination angle of its prism face 6f is .theta..sub.1, with the incident angle of .alpha..sub.1, its emitting or refractive angle .beta..sub.1 becomes such a value to satisfy the following expression. ##EQU1##
At this time, a difference .phi..sub.1 between the light propagation direction (broken line direction in FIG. 6A) where there is no prism 6 and the light propagation direction where the prism 6 exists is expressed as follows: EQU .phi..sub.1 =.beta..sub.1 -.alpha..sub.1 ( 2)
This difference .phi..sub.1 causes the displacement of P.sub.1 Q.sub.1 on the phosphor face 1a.
Next, as shown in FIG. 6B, if the light from the screen 4 is introduced to the similar prism face 6f at the incident angle of .alpha..sub.2 (&gt;.alpha..sub.1), its refractive angle .beta..sub.2 satisfies the following expression. ##EQU2##
At this time, a difference .phi..sub.2 between the light propagation direction (broken line direction in FIG. 6B) where there is no prism 6 and the light propagation direction where the prism 6 exists is expressed as follows: EQU .phi..sub.2 =.beta..sub.2 -.alpha..sub.2 ( 4)
This difference .phi..sub.2 causes the displacement of P.sub.2 Q.sub.2 on the phosphor face 1a.
If the expressions (1) and (2) are expressed by a general expression with the incident angle .alpha. and the refractive angle .beta., the following equation is obtained. ##EQU3##
From the equation (5), the refractive angle .beta. can be obtained as follows: ##EQU4##
Thus, the difference .phi. between the light propagation directions in cases of prism 6 and no prism can be expressed as follows: ##EQU5##
Now, if, for example, n1=1.531, n2=1.490 and the incident angle .alpha. is varied as 10.degree., 30.degree., 45.degree. and 60.degree., the value of .phi. becomes as expressed in the following table.
______________________________________ .alpha. (degree) .phi. (degree) ______________________________________ 10 2.78 .times. 10.sup.-1 30 9.14 .times. 10.sup.-1 45 1.598 60 2.853 ______________________________________
From the above table it will be understood that since .alpha..sub.2 &gt;.alpha..sub.1, .phi..sub.2 &gt;.phi..sub.1 and accordingly, P.sub.2 Q.sub.2 &gt;P.sub.1 Q.sub.1. Therefore, it is understood that as the incident angle .alpha. of the light to the prism face 6f becomes large, the displacement on the phosphor face 1a for the case of no prism 6 becomes large.
The above description is made on the case where the light from the predetermined portion on the screen 4 is introduced to the phosphor face 1a through the prism 6, by way of example. However, in the case that the propagation direction of light is opposite, namely the image light from the phosphor face 1a is introduced to the screen 4 through the prism 6, the similar displacement is generated.
By the way, now respective points forming the prism face 6f are taken into consideration. The image lights incident from the central portion S and peripheral portions L and R of the phosphor face 1a to the above respective points have incident angles with predetermined great and small relation always. In FIG. 5, for example, on the prism face 6a, the incident angle of the light thereto from the peripheral portion R is relatively large as compared with that of the light from the central portion S, while on a prism face 6b the incident angle of the light from the peripheral portion L is relatively large as compared with that of the light from the central portion S. Therefore, the displacements of the image lights from the central portion S and the peripheral portions L and R of the phosphor face 1a on the screen 4 differ from one another, and consequently distortion is generated in the magnified images 5a and 5b on the screen 4 as described above.
Even when the light similar to that shown in FIG. 6B is introduced to the prism face 6f, if the inclination angle of the prism face 6f is selected as .theta..sub.2 (&lt;.theta..sub.1) as shown in FIG. 6C, the incident angle .alpha..sub.2 ' of the light to the prism face 6f can be equivalently made small as follows: EQU .alpha..sub.2 '=.alpha..sub.2 -(.theta..sub.1 -.theta..sub.2) (8)
Accordingly, by suitably selecting the angle .theta..sub.2, P.sub.2 'Q.sub.2 '=P.sub.1 Q.sub.1 can be satisfied.
As described above, according to the prior art example of FIG. 3 (FIG. 5), distortion is generated in the images 5a and 5b formed on the screen 4 and this distortion can not be corrected by this prior art example shown in FIG. 3 (FIG. 5).
That is, since the projection lens 3 and the prism 6 are located near with each other in the example of FIG. 3, through almost all areas of the prism 6, the image light from the central portion S of the phosphor face 1a of the color cathode ray tube 1 (which image light is within the portion surrounded by lines l.sub.S1 and l.sub.S2 in FIG. 5) and the image light from the peripheral portions L and R (the portions surrounded by lines l.sub.L1 and l.sub.L2 and lines l.sub.R1 and l.sub.R2 in FIG. 5) commonly pass. Therefore, even if the configuration of the prism 6 is changed, the displacement of the image light can not be made equal between the central portion S and the peripheral portions L and R.